As described in French Pat. No. 2,376,919, a manhole-cover latch comprises a lock plug that is recessed in the cover or hatch with its head exposed to the outside and its inside portion carrying a locking pawl or dog. This plug can be pivoted between a locking position with the pawl preventing the cover from being opened and a freeing position permitting such opening. In the locking position formations on the latch plug align with formations on the respective cover to allow a key to fit over the plug, and in the freeing position further formations prevent the key from being retracted, so that the cover must be locked before the key can be withdrawn.
Such an arrangement works relatively well when the manufacturing tolerances for the various parts are quite tight. When, as is common, the cover and its associated parts are somewhat crudely constructed, or when they have suffered some mechanical damage, the latch frequently becomes fully nonoperational.
In addition these devices can only be operated from the outside. Thus it is impossible for a man working in a utility duct to exit through a cover that has not been opened in advance from the outside.